The present invention relates to hard disk drive circuits. More particularly, the present invention provides a high performance read/write circuit with an adjustable impedance across the disk drive magnetic head.
General purpose computers that require mass-storage capabilities utilize magnetic disks to store large amounts of data. Data are stored by magnetizing the disk surface in one of two possible orientations to indicate either a logic one or a logic zero. This is referred to as the "write" operation. To "read" the data, a sensor detects the changes in the orientation of magnetic domains on the disk and translates them into logic ones or zeros.
The read and write operations are performed by disk drive assemblies that include an inductive head floating a fraction of a micrometer above the disk surface on an aerodynamic "slider" bearing. When writing data, the head always carries a current Iw, which creates a magnetizing force large enough to saturate the magnetic medium. The disk drive circuit keeps Iw constant when a logic zero is written on the disk, and when writing a logic one the circuit reverses the polarity of the current to switch the orientation of the magnetization on the disk. When reading data, the inductive head senses these changes in the orientation of magnetic domains on the disk which produce a voltage pulse across the inductive head according to Faraday's law. The amplitude of a read voltage pulse lies between 0.1 and 2 mV. An amplifier connects across the head to amplify the read voltage for accurate detection.
General considerations dictating the design of a disk drive circuit include frequency response and noise in the read mode and settling time in the write mode. These differing considerations result in changing requirements for the disk drive circuit in the read mode as compared to the write mode. For example, in the read mode, the parallel combination of the input capacitance of the read amplifier and the inductance of the head form a second-order low-pass filter. The peak produced by series resonance in the passband of this filter requires a damping resistor in parallel with the amplifier inputs. The value of this damping resistor is designed to optimize the circuit frequency response given the values for the head inductance and total effective capacitance that includes the amplifier input capacitance as well as other parasitic capacitances.
In the write mode, however, the read amplifier is no longer active and instead the write driver (a current switching circuit) is coupled across the head. Therefore, the total capacitance across the head does not necessarily remain constant. Furthermore, in the write mode, the damping resistor directly affects the settling time characteristics of the circuit. Because the write driver cannot instantaneously change the current through the head, the head current will undergo a transient with some characteristic settling time determined by the LRC circuit composed of the head (L), total capacitance (C), and the damping resistance (R).
Generally, a smaller damping resistor is required in the write mode compared to the read mode. To optimize the circuit performance in each mode of operation, it is therefore desirable to be able to vary the value of the damping resistor.
In the past, disk drive circuits have been designed with a compromised fixed value for the damping resistor that meets the minimum requirements of each mode. However, this is clearly not a satisfactory solution. One suggested approach to variable damping resistance includes a fixed damping resistor that is present in both modes. A combination of two back-to-back Schotcky diodes in series with a smaller resistor is then connected in parallel with the fixed resistor. The diodes effectively remove the smaller resistor from the circuit during the read operation when the voltage induced across the head is in the millivolts range (i.e. too small to turn on the diodes). In the write mode, the higher voltage across the head turns on the diodes to couple the smaller resistor in parallel with the fixed damping resistor across the head. This reduces the effective resistance to provide the required damping for the write current waveform.
This approach, however, suffers from several drawbacks. The applicability of this design is limited to those disk drive circuits that present a differential voltage greater than the turn on voltage of the diodes (approximately 0.3 v) in the write mode. Furthermore, the required additional circuit elements (a second smaller resistor and Schotcky diodes) connected across the magnetic head may cause undesirable effects. For example, the diodes introduce parasitic capacitances to only one end of the head that also vary in value depending on whether the diodes are on or off.
Therefore, an object of the present invention is to provide a read/write circuit with an adjustable damping resistor that is optimized for both read and write modes of operation. Another object of the present invention is to provide a read/write circuit with a much lower and well defined impedance across the head for all voltages in the write mode.